Authors:Imre Szilágyi, István Sajó, Péter Király, Gábor Tárkányi, Attila Tóth, András Szabó, Katalin Varga-Josepovits, János Madarász, and György Pokol
This article discusses the formation and structure of ammonium tungsten bronzes, (NH4)xWO3−y. As analytical tools, TG/DTA-MS, XRD, SEM, Raman, XPS, and 1H-MAS NMR were used. The well-known α-hexagonal ammonium tungsten bronze (α-HATB, ICDD 42-0452) was thermally reduced and
around 550 °C a hexagonal ammonium tungsten bronze formed, whose structure was similar to α-HATB, but the hexagonal channels
were almost completely empty; thus, this phase was called reduced hexagonal (h-) WO3. In contrast with earlier considerations, it was found that the oxidation state of W atoms influenced at least as much the
cell parameters of α-HATB and h-WO3, as the packing of the hexagonal channels. Between 600 and 650 °C reduced h-WO3 transformed into another ammonium tungsten bronze, whose structure was disputed in the literature. It was found that the
structure of this phase—called β-HATB, (NH4)0.001WO2.79—was hexagonal.
Authors:I. Szilágyi, J. Madarász, G. Pokol, F. Hange, G. Szalontai, Katalin Varga-Josepovits, and A. Tóth
This paper discusses the changes in the structure and thermal reduction of nanosize hexagonal ammonium tungsten bronze (HATB),
(NH4)0.33−xWO3−y, which were caused by K+ ion exchange (doping) and studied by XRD, XPS, 1H-MAS NMR, FTIR, SEM and TG/DTA-MS. Comparison of the cell parameters of undoped and doped HATB revealed that both a and c cell parameters decreased after the ion exchange reaction, which showed that smaller K+ ions partly replaced the larger NH4+ ions in the hexagonal channels of HATB. After the reaction, from the hexagonal channels less NH3 evolved, which also supported the incorporation of K+ ions into the hexagonal channels.
Authors:I. Szilágyi, J. Madarász, F. Hange, and G. Pokol
Thermal decomposition of ammonium paratungstate
tetrahydrate, (NH4)10[H2W12O42]4H2O has been followed by simultaneous TG/DTA and online evolved
gas analysis (TG/DTA-MS) in flowing 10% H2/Ar directly
up to 900C. Solid intermediate products have been structurally evaluated
by FTIR spectroscopy and powder X-ray diffraction (XRD). A previously unexplained
exothermic heat effect has been detected at 700–750C. On the basis
of TG/DTA as well as H2O and NH3
evolution curves and XRD patterns, it has been assigned to the formation and
crystallization heat of γ-tungsten-oxide (WO2.72/W18O49)
from β-tungsten-oxide (WO2.9/W20O58)
and residual ammonium tungsten bronze.
Authors:I. Szilágyi, Judit Pfeifer, C. Balázsi, A. Tóth, Katalin Varga-Josepovits, J. Madarász, and G. Pokol
We studied the thermal stability of different hexagonal tungsten trioxide, h-WO3 samples, which were prepared either by annealing hexagonal ammonium tungsten bronze, (NH4)0.33−xWO3−y, or by soft chemical synthesis from Na2WO4. The structure and composition of the samples were studied by powder XRD, SEM-EDX, XPS and 1H-MAS NMR. The thermal properties were investigated by simultaneous TG/DTA, on-line evolved gas analysis (TG/DAT-MS), SEM
and in situ powder XRD. The preparative routes influenced the thermal properties of h-WO3 samples, i.e. the course of water release, the exothermic collapse of the hexagonal framework and the phase transformations
were all affected.
Authors:Imre Miklós Szilágyi, Eero Santala, Mikko Heikkilä, Marianna Kemell, Timur Nikitin, Leonid Khriachtchev, Markku Räsänen, Mikko Ritala, and Markku Leskelä
Szilágyi , IM , Hange , F , Madarász , J , Pokol , G . In situ HT-XRD study on the formation of hexagonal ammoniumtungstenbronze by partial reduction of ammonium paratungstate tetrahydrate . Eur J Inorg Chem . 2006 ; 17 : 3413 – 3418 . 10